Over the past decade, public awareness of the harmful effects and dangers of hazardous gases in the air has increased, resulting in a growing demand for accurate, inexpensive, and compact devices that detect such gases.
Conventional battery operated, portable gas detection devices typically use electrochemical cells for sensing the presence of hazardous gases. Such units are expensive ($500 to $1000 per unit), require periodic calibration, and frequent sensor and battery replacement. Other portable gas detection devices use semiconductor based, metal oxide sensors which require large amounts of power to operate. Such units are not able to operate for reasonable periods of time (e.g. more than a few hours) on batteries. Thus, there remains an unmet need for a low cost, reliable, accurate, easy to use, low power, battery powered unit for detecting hazardous gases.
A variety of optical gas sensors for detecting the presence of hazardous gases, especially carbon monoxide ("CO"), are known. Exemplary optical gas sensors are described in U.S. Pat. Nos. 5,063,164; 5,302,350; 5,346,671; and, 5,405,583 the contents of which are hereby incorporated by reference. An improved optical gas sensor system has been made by optically combining gas sensors having a response over a wide range of humidity and temperature conditions as disclosed in U.S. Pat. No. 5,618,493, the contents of which are also hereby incorporated by reference.
Generally, optical gas sensors include a self-regenerating, chemical sensor reagent impregnated into or coated onto a semi-transparent substrate. The substrate is typically a porous monolithic material, such as silicon dioxide, aluminum oxide, aluminosilicates, etc. Upon exposure to a predetermined target gas, the optical characteristics of the sensor change, either darkening or lightening depending on the chemistry of the sensor.
Battery powered, target gas detection devices utilizing optical gas sensors are commercially available and have met with great market success. Such devices include at least one sensor placed in a light path between a light emitting means and a light detecting means. The light detecting means monitors the optical characteristics of the sensor by measuring the level of light transmitted through the sensor. Electronic components of the device are devised so that when the detected level of transmitted light falls below a predetermined fixed level, an alarm or other warning means is activated.
Beyond the activation of a warning or alarm when hazardous conditions exist, users of such hazardous gas detection devices often have a need or desire to know other relevant information such as concentration, time weighted average (TWA), total dose received in a given time period, and rate of change of the concentration of the target gas.